Severe acute respiratory syndrome (SARS), a worldwide outbreak of atypical pneumonia with an overall mortality rate of about 3 to 6%, has been attributed to a coronavirus following tests of causation according to Koch's postulates, including monkey inoculation (R. Munch, Microbes Infect 5, 69-74, January 2003). The coronaviruses are members of a family of enveloped viruses that replicate in the cytoplasm of animal host cells (B. N. Fields et al., Fields virology, Lippincott Williams & Wilkins, Philadelphia, 4th ed., 2001). They are distinguished by the presence of a single-stranded plus sense RNA genome, approximately 30 kb in length, that has a 5′ cap structure and 3′ polyA tract. Hence the genome is essentially a very large mRNA. Upon infection of an appropriate host cell, the 5′-most open reading frame (ORF) of the viral genome is translated into a large polyprotein that is cleaved by viral-encoded proteases to release several nonstructural proteins including an RNA-dependent RNA polymerase (Pol) and an ATPase helicase (Hel). These proteins in turn are responsible for replicating the viral genome as well as generating nested transcripts that are used in the synthesis of the viral proteins. The mechanism by which these subgenomic mRNAs are made is not fully understood, however transcription regulating sequences (TRSs) at the 5′end of each gene may represent signals that regulate the discontinuous transcription of subgenomic mRNAs (sgmRNAs). The TRSs include a partially conserved core sequence (CS) that in some coronaviruses is 5′-CUAAAC-3′. Two major models have been proposed to explain the discontinuous transcription in coronaviruses and arterioviruses (M. M. C. Lai, D. Cavanagh, Adv Virus Res. 48, 1 (1997); S. G. Sawicki, D. L. Sawicki, Adv. Exp. Med Biol. 440, 215 (1998)). The discovery of transcriptionally active, subgenomic-size minus strands containing the antileader sequence and transcription intermediates active in the synthesis of mRNAs (D. L. Sawicki et al., J. Gen Virol 82, 386 (2001); S. G. Sawicki, D. L. Sawicki, J. Virol. 64, 1050 (1990); M. Schaad, R. S. J. Baric, J. Virol. 68, 8169 (1994); P. B. Sethna et al., Proc. Natl. Acad. Sci. U.S.A. 86, 5626 (1989)) favors the model of discontinuous transcription during the minus strand synthesis (S. G. Sawicki, D. L. Sawicki, Adv. Exp. Med Biol. 440, 215 (1998)).
The coronaviral membrane proteins, including the major proteins S (Spike) and M (Membrane), are inserted into the endoplasmic reticulum Golgi intermediate compartment (ERGIC) while full length replicated RNA (+ strands) assemble with the N (nucleocapsid) protein. This RNA-protein complex then associates with the M protein embedded in the membranes of the ER and virus particles form as the nucleocapsid complex buds into the ER. The virus then migrates through the Golgi complex and eventually exits the cell, likely by exocytosis (B. N. Fields et al., Fields virology, Lippincott Williams & Wilkins, Philadelphia, 4th ed., 2001). The site of viral attachment to the host cell resides within the S protein.
The coronaviruses include a large number of viruses that infect different animal species. The predominant diseases associated with these viruses are respiratory and enteric infections, although hepatic and neurological diseases also occur with some viruses. Coronaviruses are divided into three serotypes, Types I, II and III. Phylogenetic analysis of coronavirus sequences also identifies three main classes of these viruses, corresponding to each of the three serotypes. Type II coronaviruses contain a hemagglutinin esterase (HE) gene homologous to that of Influenza C virus. It is presumed that the precursor of the Type II coronaviruses acquired HE as a result of a recombination event within a doubly infected host cell.
In view of the rapid worldwide dissemination of SARS, which has the potential of creating a pandemic, along with its alarming morbidity and mortality rates, it would be useful to have a better understanding of this coronavirus agent at the molecular level to provide diagnostics, vaccines, and therapeutics, and to support public health control measures.